The present invention relates to a liquid crystal display device, and particularly to an improvement of a reflection-transmission hybrid type liquid crystal display device.
Taking advantage of thin shape and low power consumption, liquid crystal display devices have been extensively used for laptop personal computers, display units for car navigation, portable information terminals (PDAs: Personal Digital Assistants), and cellular telephones. These liquid crystal display devices are generally classified into a transmission type and a reflection type. In the transmission type, image display is performed by switching the on/off state of light by a liquid crystal panel, wherein the light has been emitted from an internal light source called “backlight” and enters the liquid crystal panel, and a reflection type in which image display is performed by switching the on/off state of external light such as sun light by a liquid crystal panel, wherein the external light has been reflected from a reflection plate or the like and enters the liquid crystal panel.
The transmission type liquid crystal display device, however, has problems that since the power consumption of the backlight is as large as 50% or more of the total power consumption of the display device, the total-power consumption of the display device is increased by using the backlight, and that when the surrounding environment is bright, an image displayed by the light is dark, to degrade the visibility of the image.
The reflection type liquid crystal display device overcomes the problem associated with the increased power consumption because the display device is provided with no backlight; however, such a display device has a problem that when the surrounding environment is dark, the amount of reflected light is reduced, to significantly degrade the visibility of an image displayed by the reflected light.
To solve the problems of both the transmission type liquid crystal display device and the reflection type liquid crystal display device, there has been proposed a reflection-transmission hybrid type liquid crystal display device intended to realize both the transmission display mode and the reflection display mode by one liquid crystal panel. In this reflection-transmission hybrid type liquid crystal display device, when the surrounding environment is bright, display (reflection display) is performed by using external light reflected from a reflection plate or the like, and when the surrounding environment is dark, display (transmission display) is performed by using light emitted from a backlight. Such a reflection-transmission hybrid type liquid crystal display device has been disclosed, for example, in Japanese Patent No. 2955277 and Japanese Patent Laid-open No. 2001-166289.
FIG. 6 is a plan view showing a planar structure of a thin film transistor (hereinafter, referred to as “TFT”) substrate 102 of a related art reflection-transmission hybrid type liquid crystal display device 101. Referring to this figure, a plurality of pixel electrodes 103 controlled by TFTs (to be described later) are arranged in a matrix on the TFT substrate 102, and gate lines 104 for supplying scanning signals to the TFTs and data lines 105 for supplying display signals to the TFTs are provided in perpendicular to each other on the TFT substrate 102 in such a manner as to surround the pixel electrodes 103, to form pixel regions.
Auxiliary capacitance lines (hereinafter, referred to as “Cs lines”) 106 made from a metal film are provided on the TFT substrate 102 in such a manner as to be in parallel to the gate lines 104. As will be described later, the Cs line 106 forms an auxiliary capacitance C between a connection electrode and the same, and is connected to a counter electrode provided on a color filter substrate.
A reflection display region A for reflection display and a transmission display region B for transmission display are provided in each of the pixel electrodes 103.
FIG. 7 shows a cross-sectional structure of the liquid crystal display device 101 along line F—F′ of FIG. 6. The liquid crystal display device has a structure that the above-described TFT substrate 102 and a color filter substrate 107 are disposed in such a manner as to face to each other with a liquid crystal layer 108 held therebetween.
The color filter substrate 107 has a structure that a color filter 110 and a counter electrode 111 made from ITO (Indium Tin Oxide) or the like are arranged in this order on a surface, facing to the TFT substrate 102, of a transparent insulating substrate 109 made from glass or the like. The color filter 110 is a resin layer portion colored into respective colors by pigments or dyes, and is typically composed of a combination of filter layers of colors of R (red), G (green), and B (blue).
A quarter wavelength (λ/4) layer 112 and a polarizing plate 113 are arranged on a surface, opposed to the surface provided with the color filter 110 and the counter electrode 111, of the color filter substrate 107.
In the reflection display region A of the TFT substrate 102, there are formed TFTs 115, a scattering layer 116, a planarizing layer 117, and a reflection electrode 119 on a transparent insulating substrate 114 made from a transparent material such as glass. The TFTs 115 function as switching elements for supplying display signals to the pixel electrodes 103. The scattering layer 116 is formed on the TFTs 115 via a multi-layer insulating film (to be described in detail later). The planarizing layer 117 is formed on the scattering layer 116. The reflection electrode 119 is formed on the planarizing layer 117 via an ITO film 118a. 
The TFT 115 shown in FIG. 7 is of a so-called bottom gate structure including a gate electrode 120, a gate insulating film 121, and a semiconductor thin film 122. The gate electrode 120 is formed on the transparent insulating substrate 114. The gate insulating film 121 is composed of a multi-layer film having a silicon nitride film 121a and a silicon oxide film 121b stacked on the upper surface of the gate electrode 120. The semiconductor thin film 122 is formed on the gate insulating film 121, wherein regions, on both sides of the gate electrode 120, of the semiconductor thin film 122 are taken as N+ diffusion regions. The gate electrode 120 is formed by extending part of the gate line 104, and is made from a metal such as molybdenum (Mo) or tantalum (Ta) or an alloy thereof by sputtering or the like.
A contact hole is formed in both a first interlayer insulating film 123 and a second interlayer insulating film 124 at a position corresponding to that of one of the N+ diffusion regions of the semiconductor thin film 122. A source electrode 125 is-connected to the one of the N+ diffusion regions of the semiconductor thin film 122 via the contact hole. The data line 105 is connected to the source electrode 125. A data signal is inputted to the source electrode 125 via the data line 105. Another contact hole is formed in both the first interlayer insulating film 123 and the second interlayer insulating film 124 at a position corresponding to that of the other of the N+ diffusion regions of the semiconductor thin film 122. A drain electrode 126 is connected to the other of the N+ diffusion regions of the semiconductor thin film 122 via the contact hole. The drain electrode 126 is connected to a connection electrode 127, and is electrically connected to the pixel electrode 103 via a contact portion 128. The connection electrode 127 forms the auxiliary capacitance C between the Cs line 106 and the same via the gate insulating film 121. The semiconductor thin film 122 is made from low temperature polysilicon, for example, by a CVD (Chemical Vapor Deposition) process. The semiconductor thin film 122 is formed at a position aligned with that of the gate electrode 120 via the gate insulating film 121.
A stopper 129 is provided directly over the semiconductor thin film 122 via the first interlayer insulating film 123 and the second interlayer insulating film 124. The stopper 129 is adapted to protect the semiconductor thin film 122 formed at the position aligned with that of the gate electrode,120.
In the transmission display region B of the TFT substrate 102, various insulating films formed substantially over the entire surface of the reflection display region A, that is, the gate insulating film 121, the first interlayer insulating film 123, the second interlayer insulating film 124, the scattering layer 116, and the planarizing layer 117 are removed, and a transparent electrode 118 is directly formed on the transparent insulating substrate 114. The reflection electrode 119 formed in the reflection display region A is not formed in the transmission display region B, either.
Like the color filter substrate 107, a λ/4 layer 130 and a polarizing plate 131 are disposed in this order on a surface, on the side opposed to that provided with the TFTs 115 and the like, of the TFT substrate 102, that is, on the side provided with a backlight as an internal light source (not shown), of the TFT substrate 102.
In the related art reflection-transmission hybrid type liquid crystal display device 101 having the above-described configuration, high quality image display can be realized in either the reflection display mode or the transmission display mode because the thickness of the liquid crystal layer 108 in the reflection display region A is different from that of the liquid crystal layer 108 in the transmission display region B.
A difference-in-height between the reflection display region A and the transmission display region B in each pixel region on the TFT substrate 102 is typically set to about 2 μm. As shown in FIG. 7, such a difference-in-height portion has a sharp gradient, to cause problems that liquid crystal domains are liable to occur at a boundary region (equivalent to the difference-in-height portion) between the reflection display region A and the transmission display region B, and that since a gap (thickness of the liquid crystal layer) at the difference-in-height portion satisfies neither a gap required for reflection display nor a gap required for transmission display, the difference-in-height portion contributes neither reflection display nor transmission display, whereby leakage of light may occur at the difference-in-height portion. The region contributing to neither reflection display nor transmission display is hereinafter referred to as “ineffective region”. The ineffective region degrading the display quality is generally required to be shielded by a shield film or the like.
By the way, in recent years, to realize more highly precise image display, there has been proposed a liquid crystal display device having a structure that the transmission display region B contributing to transmission display is broadened as shown in FIG. 8.
As a result of broadening the transmission display region B, in each pixel region surrounded by the data lines 105 and the gate lines 104, sections, positioned on both sides of the transmission region A in the direction parallel to the gate line, of the reflection display region A are relatively narrowed. As a result, the transmission display region B becomes close to the data line 105, as shown in FIG. 8, in the direction parallel to the gate line 104 (horizontal direction in the figure).
In the case where the transmission display region B is separated apart from the data line 105 as shown in FIG. 6, the difference-in-height between the reflection display region A and the transmission display region B in each of the direction parallel to the data line (vertical direction in FIG. 6) and the direction parallel to the gate line (horizontal direction in FIG. 6) is, as shown in FIG. 7, equivalent to the total of the thicknesses of the gate insulating film 121, the first interlayer insulting film 123, the second interlayer insulating film 124, the scattering layer 116, the planarizing layer 117, and the reflection electrode 119. On the other hand, in the case where the transmission display region B becomes close to the data line 105 as shown in FIG. 8, the difference-in-height between the reflection display region A and the transmission display region B in the direction parallel to the data line (vertical direction) is the same as that described above; however, the difference-in-height between the reflection display region A and the transmission display region B in the direction parallel to the gate line (horizontal direction) substantially becomes the difference-in-height between the data line region and the transmission display region B because the section, between the transmission display region B and the data line 105, of the reflection display region A is very narrow. By the way, the thickness of the data line region is a value obtained by adding the thickness of the data line 105 to the above-described total thickness. Accordingly, the difference-in-height between the data line region and the transmission display region B in the direction parallel to the gate line (horizontal direction) becomes large, with a result that it fails to obtain a gap (thickness of the liquid crystal layer) required for transmission display at such a difference-in-height.
As a result, the ineffective region becomes large in the section, adjacent to the data line 105, of the transmission display region B, and thereby an effective region becomes relatively small, to cause a problem that desired brightness cannot be obtained in the transmission mode, although the transmission display region B is broadened.